1. Field of the Invention
The present invention relates to a polarization hologram lens for an optical pickup, the optical pickup, and an information reproduction apparatus or an information recording apparatus having the optical pickup, which are used for compatibly reproducing or recording information from or on optical discs having different substrate thicknesses or different recording densities.
2. Description of the Related Art
In recent years, as optical discs on which information is recorded, there has been developed various information storage media, such as DVD (Digital Video Disc or Digital Versatile Disc) and the like, whose recording density is improved compared with that of CD (Compact Disc) conventionally used. Therefore, the need for an information reproduction apparatus or an information recording apparatus capable of both reproducing and recording information from and on various kinds of optical discs is increased, and the optical pickup to be used in these apparatuses requires the compatibility in recording and reproduction for CD and DVD.
Incidentally, since the DVD has higher recording density than that of the CD and a thin substrate thickness, large numerical aperture is required. On the contrary, since the CD has a thick substrate thickness, its substantial numerical aperture must be small to avoid such a possibility that various servo signals and information signals are deteriorated. Therefore, in order to switch large numerical aperture and small numerical aperture without changing the arrangement of optical parts of the optical pickup, there has been proposed a technique utilizing a polarization hologram lens.
In one example of such technique, an optical pickup constituted such that a light beam passes through a polarization hologram lens, which is provided with a grating patterns on its outer circumference and a substantially circular aperture on its center portion, to enter an objective lens. In the optical pickup having such a configuration, an ordinary ray, for example, is not diffracted by the grating patterns and directly enters the objective lens. On the contrary, an extraordinary ray is diffracted by the grating patterns at the outer circumference of the hologram lens and only the ray passed through the aperture enters the objective lens, and hence a numerical aperture can be relatively smaller. Due to such a configuration, the numerical aperture can be switched to be small or large according to different substrate thicknesses, and thus it is possible to realize the optical pickup which can be compatibly used for optical discs having different recording densities or substrate thicknesses.
However, in order to realize the above conventional optical pickup which can be compatibly used, it is necessary to prevent the diffracted light, which was diffracted by the grating pattern provided on the outer circumference of the polarization hologram lens, from entering the objective lens because if the diffracted light is focused on the optical disc, its reflected light causes a deterioration of a detecting performance. For this reason, the polarization hologram lens and the objective lens should be arranged with a given necessary spacing there between. This necessarily makes the optical pickup be larger in size and also makes it difficult to miniaturize a movable portion of an actuator and to reduce its weight, thereby increasing a power consumption.
In addition, in order to avoid such a problem, an optical pickup can be constituted such that a grating pattern of a polarization hologram lens is blazed to suppress the generation of a xe2x88x921st order diffracted light, thereby shortening the spacing between the polarization hologram lens and objective lens to a certain degree. However, when the grating pattern is blazed, the manufacture of the polarization hologram lenses becomes very difficult, and thus a great rise of the cost of the optical pickup cannot be avoided.
As mentioned above, there has been such a problem that it is difficult to provide the optical pickup, for a compatible use in information storage media having different recording densities and/or different substrate thicknesses, which has a miniaturized size, a reduced weight, a suppressed power consumption and a reduced cost.
The present invention has been achieved with such points in view, and an object is to provide an optical pickup which is miniaturized in size, light in weight and manufactured easily with low cost and which has lower power consumption.
Another object of the present invention is to provide an information reproduction apparatus and an information recording apparatus using the optical pickup, and to provide a polarization hologram lens for use as a component of the optical pickup.
According to one aspect of the present invention, there is provided a polarization hologram lens including: a first portion, positioned at a center of the hologram lens, on which no grating pattern is formed; and a second portion, positioned at an outer circumferential area of the first portion, on which grating patterns are formed, wherein the grating patterns do not diffract a light beam of a first polarization direction, and wherein the grating patterns diffract a light beam of a second polarization direction perpendicular to the first polarization direction to produce a diverging diffracted light or a converging diffracted light.
In accordance with the polarization hologram lens thus configured, when the light beam of the first polarization direction is supplied, it directly passes through both its first and second portions. On the other hand, when the light beam of the second polarization direction enters the polarization hologram lens, it directly passes therethrough at its first portion, but is diffracted by the grating patterns at the second portion. The light beam diffracted by the grating patterns becomes a diverging bundle of light beams or a converging bundle of light beams.
Therefore, the area through which the light beam passes is changed according to a polarization direction of the light beam entered the polarization hologram lens, and the diffracted light from the grating patterns is diverged or converged. For this reason, in the case where the objective lens is positioned at the backward stage, the diffracted light is not overlapped on the light beam which passed through the first portion and is not converged in a plane where the non-diffracted light beam is converged.
The grating patterns may include plural arc patterns produced from a part of concentric circular grating patterns defined around a center position which has a predetermined offset amount from the center of the concentric circular grating patterns.
With this arrangement, when the light beam of the first polarization direction enters the polarization hologram lens, it directly passes at the first and the second portions. On the contrary, when the light beam of the second polarization direction enters the polarization hologram lens, it directly passes at the first portion, but it is diffracted at the second portion by the arc patterns. Therefore, since the diffracted light due to the arc patterns of the polarization hologram lens are diverged or converged at the predetermined diffraction angle, if the objective lens is position at the backward stage, the diffracted lights have the focused points shifted from the focal point of the light beam passed through the first portion.
According to another aspect of the present invention, there is provided an optical pickup, including: the polarization hologram lens mentioned above; a light source for emitting a light beam; a polarization control unit for switching a polarization direction of the light beam to supply the light beam of one of the first polarization direction and the second polarization direction to the polarization hologram lens; an objective lens for converging the light beam passed through the polarization hologram lens to be incident on an information recording surface of an information storage medium; and a light receiving element for receiving a reflected component of the light beam reflected from the information recording surface, wherein a focus position of the diffracted light produced by the polarization hologram lens on the information recording surface is out of a range of a depth of focus with respect to the information recording surface.
According to the optical pickup thus configured, the light beam emitted from the light source is controlled by the polarization control unit by switching the polarization direction, and the light beam enters the polarization hologram lens. After the light beam passed through the polarization hologram lens enters the objective lens and is converged on the information recording surface, its reflected light is received by the light receiving element. At this time, the diffracted lights due to the polarization hologram lens are converged on a portion out of the range of the depth of focus with respect to the information recording surface of the information recording medium via the objective lens.
Therefore, since the area where the light beam directly passes through the polarization hologram lens can be changed according to the switching by the polarization control unit, the numerical aperture can be changed to be small or large according to the substrate thickness and the like. Moreover, even if the diffracted lights produced by the grating patterns are converged by the objective lens, they are in defocus state on the surface of the information recording medium, and thus an influence of the reflected light which exerts on the detecting performance is reduced. As a result, the polarization hologram lens and the objective lens can be arranged adjacently to each other, thereby enabling the miniaturization of the optical pickup.
Preferably, the grating patterns may be formed so that the reflected component of the diffracted light reflected by the information recording surface does not enter the objective lens. By this, the light beam emitted from the light source is converged on the information recording medium, but when the diffracted lights produced by the polarization hologram lens are reflected by the information recording medium, they are not returned back to the objective lens, and thus their reflected lights are not received by the light receiving element. As a result, the detecting performance is not deteriorated by the influence of the diffracted light which is not originally required.
According to still another aspect of the present invention, there is provided an information reproduction apparatus for reproducing information recorded on at least two kinds of information storage media having different substrate thicknesses or different recording densities, including the optical pickup mentioned above, wherein the polarization control unit supplies the light beam of the first polarization direction to the polarization hologram lens when an information storage medium having a thin thickness substrate or a high recording density is used, and supplies the light beam of the second polarization direction to the polarization hologram lens when an information storage medium having a thick substrate thickness or a low recording density is used.
In accordance with the information reproduction apparatus thus configured, when the reproduction from an information recording medium having a thin substrate thickness or high recording density such as DVD is executed, the polarization control unit makes a control so that the light beam of the first polarization direction enters the polarization hologram lens and it is converged on the information recording medium via the polarization hologram lens with a high numerical aperture. On the other hand, when reproduction from an information recording medium having a thick substrate thickness or a low recording density such as CD is executed, the polarization control unit makes a control so that the light beam of the second polarization direction enters the polarization hologram lens and it is converged on the information recording medium via the polarization hologram lens with a relatively low numerical aperture.
Therefore, a unique information reproduction apparatus can be used for two kinds of the information storage media having different substrate thicknesses and recording densities by switching, and the optical pickup to be used for this can change the numerical aperture easily according to the type of the information storage media. Moreover, since the optical pickup is small and light, an increase in the consumption power due to the enlargement of the movable portion can be avoided.
According to still another aspect of the present invention, there is provided an information recording apparatus for recording information on at least two kinds of information storage media having different substrate thicknesses or different recording densities, including the optical pickup mentioned above, wherein the polarization control unit supplies the light beam of the first polarization direction to the polarization hologram lens when an information storage medium having a thin thickness substrate or a high recording density is used, and supplies the light beam of the second polarization direction to the polarization hologram lens when an information storage medium having a thick substrate thickness or a low recording density is used.
In accordance with the information recording apparatus thus configured, when information is recorded on an information recording medium having a thin substrate thickness or high recording density such as DVD, the polarization control unit makes a control so that the light beam of the first polarization direction enters the polarization hologram lens and it is converged on the information recording medium via the polarization hologram lens with a large numerical aperture. On the other hand, when information is recorded on an information recording medium having a thick substrate thickness or a low recording density such as CD, the polarization control unit makes a control so that the light beam of the second polarization direction enters the polarization hologram lens and it is converged on the information recording medium via the polarization hologram lens with a relatively small numerical aperture. At this time, the diffracted lights produced by the polarization hologram lens are in the defocus state on the information recording medium, and since the intensity of the light can be weakened, an error in recording by the phase change recording technique, for example, may be avoided.
Therefore, a unique information recording apparatus can be used commonly for two kinds of the information storage media having different substrate thicknesses and recording densities by switching, and the optical pickup to be used for this can change the numerical aperture easily according to the information storage media. Moreover, since the optical pickup is small and light, an increase in the consumption power due to the enlargement of the movable portion can be avoided.
According to still another aspect of the present invention, there is provided a polarization hologram lens for use in an optical pickup, including: a circular central portion for passing a light beam of both first polarization direction and a second polarization direction perpendicular to the first polarization direction without diffraction; and an outer portion surrounding the circular central portion for passing the light beam of the first polarization direction without diffraction and passing the light beam of the second polarization direction with diffraction to produce diffracted light beams.
The outer portion may include a plurality of arc-shaped grating patterns aligned with each other, and the arc-shaped grating patterns may be produced as a part of plurality of grating patterns in a form of concentric circles.
According to still another aspect of the present invention, there is provided an optical pickup including: a light beam producing unit for producing a light beam of a first polarization direction and a light beam of a second polarization direction; a switching unit for outputting the light beam of one of the first and the second polarization directions; and a lens system, including a polarization hologram lens and an objective lens, for converging the light beam outputted by the switching unit on an information recording surface of an information storage medium, wherein the lens system converges the light beam of the first polarization direction on the information recording surface in a focused manner with a first numerical aperture, and wherein the lens system converges the light beam of the second polarization direction in a focused manner with a second numerical aperture smaller than the first numerical aperture.
The lens system may diffract a part of the light beam of the second polarization direction to produce a diffracted light and converges the diffracted light on the information recording surface in a defocused manner. In a preferred embodiment, the light beam producing unit may include a light source for emitting a light beam, and a polarization unit for polarizing the light beam from the light beam source to selectively produce the light beam of one of the first polarization direction and the second polarization direction. Alternatively, the light beam producing unit may include a first light source for emitting the light beam of the first polarization direction and the second light source for emitting the light beam of the second polarization direction.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below.